1. Technical Field
The present invention relates to a signal filtering device, and more particularly, to a device for controlling a frequency response using impedance scaling.
2. Discussion of the Related Art
In general, signal filtering devices having a particular frequency response include an impedance component such as a resistor, an inductor, or a capacitor. An impedance component having a high value is typically larger than an impedance component having a low value and can become burdensome to incorporate into an integrated circuit (IC). As a result, an impedance component having a high value typically increases product cost and causes high parasitic capacitance that may result in signal distortion or attenuation.
One such example of an IC using an impedance component having a high value (e.g., a high value resistor) is a direct current (DC) offset removal circuit, which can be found in direct conversion receivers used in mobile communication systems.
Since a DC offset may saturate a baseband output terminal in a direct conversion receiver, it should be removed by a high-pass filter, containing a resistor and a capacitor. In most cases, however, a signal received from a baseband output terminal has information at a frequency of about 0. Thus, the high-pass filter has a low cutoff frequency.
FIG. 1 illustrates a general first-order resistor-capacitor (RC) high-pass filter. In order for the high-pass filter to have a low cutoff frequency, the capacitor C1 and/or the resistor R1 must be large. The large sizes of the capacitor C1 and the resistor R1 increase the size of a semiconductor chip on which the general high-pass filter is located. The resulting large chip size is disadvantageous for at least two reasons: (1) integration problems resulting from its increased size; and (2) reduction in the received signal due to high parasitic capacitance at an output node of the high-pass filter.
FIG. 2 illustrates a general active RC low-pass filter. The frequency response of the low-pass filter of FIG. 2 can be changed by using two resistors R1, R2 and a capacitor C1. In order for the low-pass filter of FIG. 2 to obtain a desired frequency response, the resistors R1, R2 and the capacitor C1 should be maintained at desired levels. In general, a resistor's value and a capacitor's value change depending on a manufacturing process or temperature. In a semiconductor chip, for example, resistance changes 30–100% and capacitance changes 10–30% due to changes in the manufacturing process, pressure, and temperature. Thus, it is necessary to tune the resistance or the capacitance to obtain the desired frequency response from the filter. One example circuit that can be used to tune capacitance is shown in FIG. 3. As shown in FIG. 3, the capacitance of a capacitor is controlled by a capacitor array that can be switched.
As shown in FIG. 3, a plurality of capacitors C11–C1n are connected in parallel with each other, and switches SW1–SWn, which are serially connected to the capacitors C11–C1n, are switched on or off. Thus, enabling the capacitance of the circuit to be controlled via a switching operation. However, this method requires additional components such as the switches SW1–SWn and several control bits B1–Bn, which are used to control the switches SW1–SWn. As a result, the circuit becomes complex, and the tuning accuracy and the range of the capacitance are defined by the number of capacitors and control bits.
A common resistance and/or capacitance controlling circuit can become complex, thus making it difficult to control its frequency response. In addition, if the filter in a resistance and/or capacitance controlling circuit requires a high resistance or a plurality of capacitors, the IC on which the circuit is located increases in size.
Accordingly, there exists a need for a device that can easily control the frequency response of a circuit by scaling the impedance.